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A security researcher on Sunday published a working exploit of a critical Windows vulnerability, making it more likely that attacks will spread.

According to a security advisory issued Friday by Microsoft, hackers can use a malicious shortcut file, identified by the ".lnk" extension, to automatically run their malware simply by getting a user to view the contents of a folder containing the shortcut. Malware can also automatically execute on some systems when a USB drive is plugged into the PC.

All versions of Windows, including the just-released beta of Windows 7 Service Pack 1 (SP1), as well as the recently retired Windows XP SP2 and Windows 2000, contain the bug.

Sunday, a researcher known as "Ivanlef0u" published proof-of-concept code to several locations on the Internet. Later that day, Belgian researcher Didier Stevens -- who in late March revealed a serious design flaw in Adobe's PDF document format -- confirmed that Ivanlef0u's code could be tweaked to create an effective attack.

Stevens also announced that he'd tested Ivanlef0u's exploit against a tool he'd written a year ago, and said that the utility successfully blocked attacks launched from USB flash drives and CDs. "You can use Ariad if you want to mitigate attacks with these shortcut links until Microsoft releases a patch," Stevens said of the tool in a Sunday blog.

In the blog post, Stevens illustrated how to set up Ariad to block executable files, including .lnk files, from running from a USB or CD drive. He also urged users to read Ariad's online documentation, and warned them that running it could be risky. "Ariad is a mini-filter drive, and as such operates inside the Windows kernel," Stevens said. "Bugs in kernel software can have grave consequences: the dreaded BSOD [Blue Screen of Death]. So please test this software first on a test machine you can miss."

Microsoft's defensive advice thus far has been limited to recommending that users edit the Windows registry to disable the displaying of all shortcut icons, and to switch off the WebClient service.

Another researcher didn't think much of Microsoft's workarounds. "This is highly impractical for most environments," argued Chester Wisniewski, a senior security advisory with Sophos. "While it would certainly solve the problem, it would also cause mass confusion among many users and might not be worth the support calls," he said. "Microsoft also suggests disabling the WebClient service that is used for WebDav. If you are not a Microsoft SharePoint customer this may be a solution, but many organizations rely on SharePoint so this is limiting as well."

The U.S. Computer Emergency Readiness Team (US-CERT) added an alert of its own Saturday and pointed out that USB flash drives were a likely, and dangerous, attack vector. "Depending on the operating system and AutoRun/AutoPlay configuration, exploitation can occur without any interaction from the user," said US-CERT.

The team also urged Windows users to disable AutoRun and AutoPlay, two Windows functions that have long been used by attackers to commandeer PCs. AutoPlay is disabled by default for removable drives on Windows 7 and Server 2008 R2, but is on by default on other versions going back to Windows XP SP2.

Complicating matters is the fact that Microsoft dropped Windows XP SP2 from support last Tuesday, meaning that when it does produce a patch, it will not deliver the fix to PCs running XP SP2.

Several security experts contended over the weekend that XP SP2 was vulnerable to attack, even though Microsoft did not specifically list it or Windows 2000, the other edition retired from support last week, as affected.

"Noticeably absent from the list are Windows 2000 and Windows XP SP2 as they are no longer supported," said Wisniewski. "They are, however, definitely still vulnerable."

In its advisory, Microsoft stuck to its policy of not naming Windows editions that it no longer supports and so does not test. Instead, the company again pressed customers to upgrade from Windows XP SP2 to SP3, or from either Windows 2000 or XP SP2 to the newer Windows Vista or Windows 7.

Stevens suggested that his Ariad tool might be the long-term solution for XP SP2 users. "As it is expected that Microsoft will not release a patch for Windows XP SP2, Ariad can offer permanent mitigation," he said.